To gears that are lubricated by splashing lubricating oil, such as gears used in motor vehicle transmissions, lubricating oil is not sufficiently supplied to their engaging portions, so that abrasion and scoring tend to develop on their tooth faces. To solve this problem, the present applicant proposed to improve the resistance to abrasion and scoring by randomly forming a multitude of microscopic recesses on the tooth face which serve to keep oil therein, thereby promoting the formation of an oil film on the tooth face (see e.g. patent document 1.).
In one specific method of randomly forming microscopic recesses in a surface of a part such as a tooth face, recesses and protrusions are first formed randomly on the surface by a special polishing method such as centrifugal fluid barreling, and then only the protrusions are flattened by barreling, thereby randomly forming microscopic recesses.
With increases in the numbers of steps in transmissions as well as increasing outputs of engines, gears used in transmissions are subjected to larger loads and rotated at higher speeds. Furthermore, for fuel economy of motor vehicles, smaller transmissions are used. Thus, smaller gears or smaller modular gears are required for use in such transmissions. For these reasons, gears used in today's transmissions tend to suffer fatigue damage such as pitting and chipping.
In order to increase the fatigue strength of machine parts such as gears, it is known to subject such parts to shot peening before finishing their surfaces by e.g. polishing, thereby producing a large compressive residual stress on the surface.    Patent Document 1: JP Utility Model Registration Publication 2591616